1. Field of the Invention
The present invention relates to a thin film transistor-liquid crystal display (TFT-LCD), and in particular to an improved low power gate driver circuit of the TFT-LCD using an electric charge recycling technique.
2. Background of the Related Art
As shown in FIG. 1, a conventional TFT-LCD includes a liquid crystal panel 10 having a plurality of pixels 10' which are formed within each intersecting areas of the gate lines GL and the data lines DL. A data driver 20 outputs a picture signal to the liquid crystal panel 10 through the data lines DLn, and a gate driver 30 turns on the pixels 10' by driving the gate lines GLn. Each pixel 10' includes a thin film transistor 1, and a storing capacitor Cs and a liquid crystal capacitor Clc. Each capacitor is connected in parallel with the thin film transistor 1.
During operation, a shift resistor (not shown) of the data driver 20 sequentially receives a picture data by a single pixel, and stores the picture data of the data lines DL. The gate driver 30 outputs a signal having the waveform shown in FIG. 2, thus sequentially driving the plurality of gate lines GLn. Each gate line GL can be modeled as a resistor and a capacitor. A size of the resistor and capacitor varies depending on a screen size and a constituent material of the gate line. Generally, the resistor has a resistance of about few K.OMEGA. (kilo ohms), and the capacitor has a capacitance of about few pF (pico farads).
In an office automation (O/A) application such as computer displays, the gate driver 30 outputs a signal having the waveform shown in FIG. 2A, and in an audio/video (A/V) application such as TVs, the gate driver 30 outputs a signal having the waveform shown in FIG. 2B of an even number field, and a signal having the waveform shown in FIG. 2C of an odd number field to drive the gate lines GL.
In a sequential scanning method for the O/A application, the gate driver 30 charges the capacitances (not shown) of the gate lines GLn in accordance with a signal having the same pattern as shown in FIG. 2A, and discharges the electric charge to a ground (or VSS), thereby driving the plurality of gate lines GLn. In an even number field of a double line simultaneous scanning method for the A/V application, as shown in FIG. 2B, the gate driver 30 drives the plurality of gate lines GLn by repetitively applying the same signal to first and second gate lines GL1 and GL2, then applying the same signal to third and fourth gate lines GL3 and GL4. In an odd number field of the double line simultaneous scanning method for the A/V application, as shown in FIG. 2C, the gate driver 30 charges the capacitances (not shown) of the gate line GL by repetitively applying a signal to the first gate line GL1, applying the same signal to the second and third gate lines GL2 and GL3, and applying the same signal to the fourth and fifth gate lines GL1 and GL5, and discharges the electric charge to the ground (or VSS) to drive the gate lines GLn.
As a result, the plurality of thin film transistors connected with the selected gate lines GLn are turned on, and picture data stored in the shift resistors (not shown) of the data driver 20 are applied to the thin film transistors to display the picture on the liquid crystal panel 10. The above-described operation is repeated to display the picture on the liquid crystal panel 10.
Generally, the output signal of the gate driver 30 swings from VDD to VSS (or the ground), or from VSS to VDD. If the gate driver 30 drives an nth gate line GLn, the power P.sub.1, which the gate driver 30 consumes, is same as the following formula (1). EQU P.sub.1 =VDD.multidot.I.sub.av =VDD.multidot.(C.sub.n .multidot.V.sub.swing .multidot.Frame frequency) (1),
where
Cn is a the capacitance of capacitor of the nth gate line GLn and I.sub.av is an average current, and V.sub.swing is a voltage swing of scanning pulse.
Accordingly, in the conventional TFT-LCD driving circuit, the gate driver 30 outputs a signal which swings from VDD to Vss (or the ground), or from VSS to VDD in order to charge/discharge the capacitance of the gate line GL, thereby consuming the power proportional to a value of VDD multiplied by V.sub.swing during the charging/discharging process.